Fix Android arm64 runtime crash: SIGSEGV in Thread::LinkedList + GC stub calloc

[zendrx]

Problem

After fixing the GC_stackbottom linker error (PR #42), the app now crashes at runtime on Android arm64 with:

signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x18
 Cause: null pointer dereference
    #00 ... (*Thread::LinkedList(Fiber)#push<Fiber>:Nil+8)
    #01 ... (*Thread::current:Thread+316)
    #02 ... (*Crystal::once ... )
    #03 ... (*Crystal::System::Env::get ... )
    ...
    #11 ... (*Native::App::current ... )  ← NilAssertionError
    #12 ... (crystal_android_main+40)

Root Cause Analysis

This is a chain crash — three separate bugs amplify each other:

Bug 1: Native::App.current is nil on Android

crystal_android_main calls Native::App.current which does @@current.not_nil!, but @@current is never set because the user's main.cr entry point (which calls App.start) doesn't execute on Android. Instead the framework jumps directly to crystal_android_main from a pthread created in native.c.

Bug 2: Crystal's Thread::current crashes on Android bionic libc

When the NilAssertionError from Bug 1 is raised, Crystal's exception handling calls Crystal::System::Env::get → Crystal::once → Thread::current. Thread::current tries to push a Fiber to the thread's linked list, but Crystal's thread-local storage isn't properly initialised on Android's bionic libc from a non-Crystal pthread → SIGSEGV.

Bug 3: GC stub uses malloc instead of calloc

Boehm GC's GC_malloc zeroes allocated memory, which Crystal relies on for object initialization (unset pointers must be null). The previous GC stub used plain malloc which leaves memory uninitialised, causing random garbage pointer dereferences.

Fixes

1. App-subclass registration (src/native/framework/app.cr)

• Adds App.registered_subclass / App.registered_subclass= so users can declare their app class from main.cr
• Adds App.start_registered which starts the registered subclass (used by Android bridge)
• Backwards-compatible — desktop builds calling App.start() directly still work

2. Android bridge fix (src/native/engine/android/bridge.cr)

• crystal_android_main now calls App.start_registered instead of App.current
• Wrapped in begin/rescue with __android_log_print so future errors go to logcat instead of producing an opaque native crash

3. GC stub calloc fix (src/native/cli/build.cr)

• GC_malloc, GC_malloc_atomic, and GC_malloc_uncollectable now use calloc(1, size) instead of malloc(size) to match Boehm GC's zeroing behaviour
• GC_realloc zeroes new memory when the pointer moves

4. PCRE2 cross-compile (carried from PR #42)

• Builds PCRE2 10.42 from source for aarch64-linux-android since Crystal's stdlib regex support requires it

User-facing changes

Your main.cr should now register the app class for Android:

class MyApp < Native::App
  def setup
    # your app setup
  end
end

# On Android: register the subclass (the framework starts it)
# On desktop: start directly as before
{% if flag?(:native_android) %}
  Native::App.registered_subclass = MyApp
{% else %}
  Native::App.start(MyApp)
{% end %}

Gradle version is not modified.